Antigen specific T-cell factors are most likely the solubilized T-cell antigen receptor, since it is unlikely that the T-cell has two different antigen specific molecules, the factor and a different receptor. Therefore, antigen specific T-cell factors offer a direct way to study the structure of the T-cell antigen receptor. In addition, the immunoregulatory functions of these factors provide a way of studying specific B-cell activation and T-B cooperation. Although such factors were shown to be active in vivo, preliminary experiments indicate that are not active in vitro.